A problem concerned with press units of the above type resides in the difficulty to obtain a good rotation locking function between the nut and the casing, either directly between the nut itself and the casing or via the thrust rod in rotation locking engagement with the casing. In previous press units of the above type there has been used different types of rotation locking arrangements, for instance a splines connection between the thrust rod and the casing by which longitudinal movements of the thrust rod is permitted whereas rotation is not. A drawback of this type of rotation locking arrangement is that it requires a rather long engagement distance between the casing and the thrust rod to keep down the contact pressure between the parts and to thereby provide an acceptably long service life of this splines connection. This means in turn that the length of the casing tends to be undesirably large, especially when applied on the press unit intended for use in cramped areas.
Another way of accomplishing a rotation locking means for this type of press units is to provide the nut with one or more radially extending key elements arranged to cooperate with longitudinal keyways in the casing. Known such arrangement have resulted in high manufacturing costs since they require particular shapes both of the casing and the nut. Known such arrangements have also been suffering not only from an unacceptable mechanical wear but also a less accurate guidance of the nut/thrust rod, because in many applications there are very high demands on the accuracy of the longitudinal movement of the thrust rod and hence, the rotational movement of the nut. It is difficult to obtain a play free engagement between the nut and the keyways in the casing, because sliding contact surfaces inevitably require a small but still a play between them to ensure a jam free operation and for carrying some lubricant.